The present invention relates to improvements in the treatment of waste and other combustible materials in an aqueous fluid stream, such as organic solvents, municipal sludge, toxic or contaminated products and the like.
Various wet oxidation treatment processes are disclosed in the prior art. In general, the temperature of combustible material dissolved and/or suspended in water in the presence of oxygen is increased to produce a wet oxidation reaction. The combustion reaction is exothermic and can be carried out in a reaction apparatus having a continuous heat exchanger such that the heat evolved by the combustion reaction is used to heat an influent waste stream to the desired temperature. This provides an enhanced, energy-efficient system.
The prior art has also proposed utilizing wet oxidation processes of this type in a continuous vertical heat exchange column which may be supported in a subterranean well. The depth of the well or column determines in part the hydrostatic fluid pressure which is maintained such that boiling of the fluid during the reaction process is prevented. Thus, the prior art has recognized the possibility of treating organic waste, including municipal sludge, in a deep well reaction vessel having tubes nested to form annuli in a heat exchange relation wherein combustible waste is introduced as an aqueous influent waste stream. The combustible waste in the fluid medium is one reactant and oxygen, introduced as a gas, such as air, is the other reactant. The air is injected under pressure into the downgoing influent tube in a manner to promote intense mixing and contacting and to increase the flow of reactants and products through the system. The quantity of gas injected into the influent annulus is controlled to provide the proper oxygen-to-waste ratio to bring about the desired reaction, as more fully disclosed in U.S. Pat. No. 4,272,383 to McGrew, which is assigned to the assignee of the instant application. The disclosure of the foregoing McGrew patent is incorporated herein by reference.
The influent waste or waste stream is pumped from the ground surface downwardly into the influent annulus at a controlled temperature, pressure and flow rate. The fluid waste forms a hydrostatic column having the desired pressure and temperature at a reaction zone deep within the reaction vessel. Under these conditions, the reactants react at an accelerated rate. The heated fluid and reaction products are then flowed upwardly through the effluent annulus to ground surface. As described, the influent and effluent annuli are preferably in heat exchange relation to best utilize the reaction heat. The temperature of the fluid in the reaction zone is controlled, preferably by adding or removing heat from the system. This can be achieved by placing a vertical heat exchange apparatus in the reactor assembly as shown in the disclosed preferred embodiment.
As will be understood by those skilled in the art, the overall efficiency of a downhole or deep well reactor apparatus of the general type described may be improved by using pure oxygen or oxygen-rich gas for injection rather than air because air contains only about 21 percent by volume oxygen. By increasing the amount of oxygen available to react with the combustible waste, the wet oxidation reaction enhanced. Thus, the prior art does in fact suggest the injection of oxygen-enriched air or even pure oxygen into an oxidation reaction vessel (see for example U.S. Pat. No. 3,449,247 to Bauer). The injection of substantially pure oxygen, oxygen-enriched air or other oxygen-enriched gas into the influent annulus can, however, cause an unwanted and potentially hazardous form of combustion resulting from dry spots produced in the reactor. That is, the injection of dry oxygen-enriched gases into the reactor may create dry areas of combustible material which can ignite in the dry, oxygen-rich environment. Also, when exposed to a dry, oxygen-rich gas, the tube wall itself may actually ignite. This series of events can cause catastrophic failure of the reactor.
The aforementioned Bauer patent also discloses the injection of a gas reactant at either the top or the bottom of the reactor well. Since a column of "pure" liquid has greater density and thus weighs more than the same liquid containing a gas, lowering the point of injection of the reactant gas in the influent annulus increases the fluid pumping efficiency. Unfortunately, the energy saved in lowering the point of injection of the reactant gas may be substantially offset by that required to pressurize the reactant gas to the pressure of the influent fluid at this low point of injection. The prior art does not suggest any improvement in the efficiency of the wet oxidation process resulting from lowering the point of injection of the reactant gas.
The method of the present invention provides a safe and efficient manner of introducing oxygen, oxygen-enriched air or an oxygen-rich gas directly into the influent waste stream in the reaction apparatus. In one embodiment, a composition containing substantilly pure gaseous oxygen or oxygen-rich gas as one component is injected into the influent annulus at multiple sites, including at least one injection point near the reaction zone.